1. Field of the Invention
The present invention relates to an apparatus for, and method of, reproducing an optical disc driven by the CAV (Constant Angular Velocity) method.
2. Description of Related Art
To reproduce data from an optical disc, the optical disc is driven to spin by either the well-known CAV (Constant Angular Velocity) or CLV (Constant Linear Velocity) method.
The CAV method is such that data is recorded into or reproduced from an optical disc being driven to spin at a constant angular velocity. An optical disc in which data has been recorded by the CAV method (will be referred to as "CAV disc" -hereinafter) has sectors as minimum data recording/reproducing units, radially formed and is featured by a rapid access to data for recording or reproduction.
The CLV method is such that data is recorded into, or reproduced from, an optical disc being spun at a constant linear velocity in relation to a recording/reproducing head. An optical disc in which data has been recorded by the CLV method (will be referred to as "CLV disc" -hereinafter) has a constant data recording density over an area from outer perimeter to inner perimeter and is capable of recording a large amount of data. Also well known is an optical disc having guide grooves called "pre-grooves" formed thereon. When data is reproduced from such an optical disc having pre-grooves formed thereon, laser beam components from opposite edges of a pre-groove are detected from a reflected laser beam and tracking servo control can be performed so that the laser beam is irradiated to the middle between the pre-groove edges.
Further an optical disc is well known in which a slightly meandering pre-groove is formed on the basis of a wobble signal generated by modulating a predetermined center frequency. The modulated component of this wobble signal contains a physical address information of a position where this wobble signal is recorded, etc. In the optical disc recording/reproducing apparatus, the wobble signal can be detected from a signal of a difference between the laser beam components from opposite edges of the pre-groove, a so-called a pushpull signal.
If such a wobble signal is recorded in a CAV disc such mentioned above, it is detected with the optical disc spun at a constant angular velocity, whereby a modulated component can be extracted from a carrier of a predetermined center frequency.
In case such a wobble signal is recorded in a CLV disc such as mentioned above, it is detected with the disc spun at a constant linear velocity, whereby a modulated component can be extracted from a carrier of a predetermined center frequency.
It will be considered here that for a rapid access to a CLV disc, the disc is spun at a constant angular velocity as in the case of a CAV disc.
In this case, however, when a wobble signal is detected from the CLV disc being spun at the constant angular velocity, the center frequency of the detected wobble signal will vary at each radial position on the CLV disc. For example, there will be a difference of about 2.5 times in linear velocity between the inner and outer perimeters of the CLV disc. Therefore, there will be a difference of about 2.5 times in center frequency of the wobble signal between the inner and outer perimeters of the CLV disc.
To demodulate the wobble signal, the ordinary optical disc drives employs a PLL (phase locked loop) circuit, filter circuit, etc. whose operating frequencies are fixed. Therefore, it is difficult to demodulate a wobble signal whose center frequency varies.